I found it is 'more or less' obvious, at least to me, what the styles are by looking at the css file in any ebook made with W2epub. I was looking as I was not happy with one or two styles and wanted to change them, preferably at creation time.
I found the cs.xba file deep in LibreOffice uno_packages which is the file that creates the css file. With a proper text editor you can alter this file to produce your own css file (at your own risk of course.) You need to restart Writer to get the changes registered, but it is quite painless and you get your books formatted as you want them.